1. Field of the Invention
This invention relates to an oxygen sensor for detecting oxygen concentration.
2. Description of the Prior Art
Various types of oxygen sensors are hitherto known. Japanese Utility Model Publication 2-34605 discloses an oxygen sensor of a limiting current type having two internal chambers. The two chambers are each constituted of an oxygen ion conductive solid-electrolyte plate, electrodes formed on both surfaces of the solid electrolyte plate, a support, and a sealing layer that hermetically secure the solid electrolyte plate and the support at their ends. The first and second chambers holds the same support in common. The first chamber communicates with its surrounding space through a first gas diffusion controlling small orifice. The second chamber communicates with the first chamber through a second gas diffusion controlling small orifice provided on the common support. When the first small orifice has been closed up with dust, this sensor can detect an increase in the diffusion resistance at the small orifice.
Japanese Laid-open Patent Application No. 2-12049 disclosed a similar structure. When an electrode has been deteriorated because of adhesion of dust such as an oxide, this sensor can detect a decrease in limiting currents.
Japanese Laid-open Patent Application No. 60-218058 discloses a method for the self-examination by a sensor, performed when limiting currents have decreased as a result of the deterioration of an oxygen sensor. This sensor has one internal chamber. This chamber is constituted of an oxygen ion conductive solid-electrolyte plate, electrodes formed on both surfaces of the solid electrolyte plate, a support, and a sealing layer that hermetically secure the solid electrolyte plate and the support at their ends. According to this method, two different voltages are alternately applied between the electrodes. The deterioration of the sensor is detected according to changes in correlation between two limiting currents.